Method for simulating a dynamic force response and method of calibration

ABSTRACT

A method of simulating a dynamic force event comprises rotating a cam about an axis according to a rotational speed profile and biasing a follower against a surface of the cam as the cam rotates so that the follower moves on the surface and imparts a braking force against the surface. The method further measures the braking force on the rotating cam with a measurement device operatively connected to the cam, the braking force varying according to the friction between the cam surface and follower moving against one another. In another aspect of the invention, a method of calibrating a testing machine measures a braking force resisting movement of a first body over time to produce a simulated dynamic force response of the first body and compares it with a standard dynamic force response to calibrate the testing machine.

BACKGROUND OF THE INVENTION

This invention relates to dynamic force testing machines, and moreparticularly to a method for simulating a dynamic force responsecorresponding to an experimental event, such as a peel test for anadhesive-backed substrate or refastenable system (e.g., a hook-and-loopfastener system) and a method of calibration.

Dynamic force testing machines, such as tensile testing machines (i.e.,constant speed of extension machines), commonly perform experiments tomeasure particular characteristics of materials or objects. Thesemeasured characteristics may then be used for further evaluation of thematerials or objects. For example, materials may be dynamically testedon tensile testing machines to ascertain their mechanical properties.Such tests are typically performed with multiple samples of differentmaterials, creating a library of measured test data comparing differentmaterials to one another. For such libraries of data to be useful,consistent performance by the testing machine is essential. This isparticularly true for dynamic testing machines, where multiplemeasurements (e.g., force, extension displacement) are recorded overtime, generating a dynamic response of a particular characteristic, suchas force.

One such experiment is a peel test for a hook-and-loop fastener. Whenthe mating components of a hook-and-loop fastener are peeled apart, theforce required to disengage the hooks from the loops varies over time.As graphically depicted, this force typically has a sawtooth or serratedprofile that varies over time, caused by gradual increases in thepeeling force as individual hooks are plastically deformed, followed bymomentary drops in force as the hooks release from respective loops.Consistently reproducing such a sawtooth dynamic force response, or anysuch dynamic force response, is the focus of the present invention.

Conventional testing machines (e.g., tensile testing machines)performing dynamic testing have suffered from various drawbacks, mostnotably the inability to calibrate the testing machines to ensureconsistent dynamic testing. For instance, performing multiple tests onsimilar portions of material may yield variability between tests.However, determining whether such variability stems from the testingmachine or the material itself, is difficult if not impossible. Tominimize variability in the testing machines, those skilled in the artutilize calibration methods. As used herein, ‘calibration’ denotesverification of a machine's accuracy, usually with an accompanyingadjustment of the machine to minimize its error. Typical calibrationtechniques are static. ‘Static calibration’ denotes calibration of amachine where the test specimen or moving elements of the machine areeither fixed or change position slowly, such that dynamic effects uponthe machine are negligible. Because dynamic effects are not included inthe calibration, static calibration techniques cannot accuratelycalibrate the dynamic response of a particular machine. A machinecalibrated statically, yet performing dynamic tests, may or may not beperforming accurately. As such, dynamic calibration techniques may beused to better confirm the dynamic performance of a machine. ‘Dynamiccalibration’ denotes calibration of a machine where the test specimen ormoving elements of the machine change position quickly, such thatdynamic effects upon the machine are no longer negligible. Dynamiccalibration is useful when applied to a single measured characteristic,or channel (e.g., force, displacement, time), of the testing machine byitself. Beyond dynamic calibration of a single channel by itself,however, dynamic calibration may be more effectively applied to multiplechannels simultaneously. Such a multi-channel calibration not onlydynamically calibrates the individual channels, it dynamicallycalibrates their interaction with one another. Without such simultaneouscalibration of such channels, individual calibration of each channelseparately cannot account for potential changes occurring only when suchcharacteristics are measured simultaneously.

Specifically, conventional static calibration techniques used inconjunction with tensile testing machines involve only staticcalibration of the force sensing portions of the tensile testingmachines, including load cells and any associated recording circuitry ofthe machines. By moving elements of the tensile testing machine (e.g.,extending a crossbar) slowly during the static calibration, the actualdynamic movement of the tensile testing machine, as compared to thedesired dynamic movement, is not calibrated. Actual movements of thetensile testing machine must accurately match the desired movements ofthe machine, however, because movements of the machine are oftenincorporated into other measured characteristics. For example, tensiletesting machines may be used to create a force versus extension curve.Because static calibration only calibrates the ability of the tensiletesting machine to measure a single characteristic, or channel (e.g.,force, displacement), in a static condition, the measurements reportedby the tensile testing machine when both characteristics are measuredsimultaneously may be inaccurate, casting doubt over the accuracy of thecurve. Limiting the calibration to only static or dynamic calibration ofindividual characteristics by themselves does not sufficiently calibratethe machine for a dynamic test. For instance, many material propertiesare strain and strain-rate dependent, making extension displacement animportant characteristic that should be calibrated simultaneously withforce to ensure accuracy. Various ASTM standards specify accuracyrequirements for tensile testing machine measurements. The widelyaccepted ASTM E4 calibration procedure, for example, employs deadweightsor highly accurate load cells to calibrate only the force measurementand recording system of the tensile testing machine. Because no otherportion of the tensile testing machine is calibrated, however, thisprocess yields only a static calibration of a single characteristic andcannot gauge the true dynamic response of the tensile testing machine.Furthermore, many conventional tensile testing machine software programshave user selectable or configurable sampling rates and data filters fordynamic testing. Mere deadweight calibration of such machines does notensure that the machine is operating properly for a given dynamic test.In other words, applying conventional calibration methods to dynamictensile testing machines calibrates only particular individualcharacteristics of the machine separately from one another, whereassimultaneous dynamic calibration occurs while the machine performsdynamically, thereby calibrating all parts of the machine and measuredcharacteristics together (e.g., load cell and extension together).Applying such a calibration verifies the interaction of individualmeasured characteristics with one another.

There is a need, therefore, for an apparatus and method capable ofaccurately dynamically calibrating the various measurement channels of atensile testing machine, or any testing machine, simultaneously byperforming repeatable dynamic testing simulating actual experimentalevents, such as (but not limited to) the aforementioned peel tests. Forinstance, such an apparatus and method would dynamically calibrate twoor more measurements simultaneously during a simulated dynamic test toverify the accuracy of such measurements when measured togethersimultaneously. For additional detail regarding apparatus for simulatinga dynamic force response, reference may be made to the utilityapplication filed simultaneously by Peter D. Honer, Oliver P. Renier andPeter S. Lortscher, entitled APPARATUS FOR SIMULATING A DYNAMIC FORCERESPONSE, assigned to Kimberly-Clark Worldwide, Inc., the entiredisclosure of which is incorporated by reference in a manner consistentherewith.

BRIEF SUMMARY OF THE INVENTION

In general, the present invention is directed to a method of simulatinga dynamic force event comprising rotating a cam about an axis accordingto a rotational speed profile. A follower is biased against a surface ofthe cam as the cam rotates so that the follower moves on the surface andimparts a braking force against the surface. The braking force on therotating cam is measured with a measurement device operatively connectedto the cam. The braking force varies according to the friction betweenthe cam surface and follower moving against one another.

The present invention is further directed to a method of calibrating atesting machine adapted for measuring a force applied to a testspecimen, wherein the force varies over time to provide a simulateddynamic force. The method comprises moving first and second bodiesrelative to one another. The second body is biased against a contouredsurface of the first body during the movement to impart a force againstthe first body. The contoured surface is shaped to vary the forceapplied against the first body to provide the simulated dynamic force. Abraking force resisting movement of the first body is measured over timeto produce a simulated dynamic force response. The simulated dynamicforce response is compared to a standard dynamic force response tocalibrate the testing machine.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation of a tensile testing machine and anapparatus of the present invention for simulating and measuring adynamic force;

FIG. 2 is an enlarged portion of the apparatus of FIG. 1;

FIG. 3 is a right side elevation of the apparatus of FIG. 2 with afollower positioned for engagement with a cam of the apparatus;

FIG. 4 is a view similar to FIG. 3 showing the follower biased intocontact with the cam;

FIG. 5 is a view similar to FIG. 3 showing the follower disengaged fromthe cam;

FIG. 6 is an isometric of the apparatus of FIG. 1 with portions removedto emphasize the interaction of particular elements;

FIG. 7 is a plot of a simulated dynamic force response, wherein thex-axis represents extension and the y-axis represents force; and

FIG. 8 is a plot of a standard dynamic force response, wherein thex-axis represents extension and the y-axis represents force.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, apparatus of this invention for simulating adynamic force over time corresponding to an experimental event isdesignated 21 in its entirety. In one embodiment, the apparatus 21mounts on a tensile testing machine, generally designated 25, having abase 27, two uprights 29 extending up from the base and a crossbar 33extending between the uprights above the base. The crossbar 33 ismovable by a conventional mechanism along the uprights 29 in thedirections of arrow A. The construction and operation of such a tensiletesting machine 25 is well known in the art and will not be describedhere. The apparatus 21 mounts on the base 27 of the tensile testingmachine 25 via a mount 39 secured to the base. A force measurementdevice 45 (e.g., a load cell) of the tensile testing machine 25 connectsto the apparatus 21 and mounts on the crossbar 33. Other measurementdevices, such as strain gages or torque gages, are also contemplated aswithin the scope of the invention and discussed in greater detail below.The mount 39 and force measurement device 45 are conventional componentsand will not be described further here. Suffice it to say that the forcemeasurement device 45 mounts on the crossbar 33, such as by conventionalmechanical fasteners (not shown), to move along a linear path as thecrossbar moves in either of the directions of arrow A (FIG. 1). Whenused in a conventional manner to conduct a peel test for a testspecimen, such as a hook-and-loop fastener, the tensile testing machine25 would appear essentially as shown in FIG. 1, except that theapparatus 21 would be replaced by the test specimen to be tested.

The focus of the present invention is the apparatus 21, which isdepicted in greater detail in FIGS. 2-5. The apparatus 21 comprises abracket 53 for mounting the elements of the apparatus. The bracket 53has three portions, a horizontal plate 53 a secured to the mount 39, aprimary support 53 b extending up from the plate and a secondary support53 c extending up from the plate generally parallel to the primarysupport (FIGS. 1 and 2). A shaft 57 is rotatably supported by theprimary support 53 b and the secondary support 53 c for rotation aboutan axis C. A cam 61 is rotatably coupled to the shaft 57 for conjointrotation with the shaft about axis C. The cam 61 may be attached to theshaft 57 in any number of ways, such as by a set screw, welding, asplined coupling or other suitable mechanism.

In the preferred embodiment, the apparatus 21 includes a drive devicecomprising a linear-to-rotary drive mechanism, generally indicated at71, operable to rotate the cam 61 in response to linear movement of theforce measurement device 45. The preferred drive mechanism 71 includes adriving cord 75 (a cable, wire or other flexible line) connected at oneend to the force measurement device 45 and at its opposite end to adrive pulley 79 rotationally coupled to the cam 61 and shaft 57. A guideroll 83 mounted on a shaft secured to the upper end of support 53 bguides the driving cord 75 such that the cord extends upward from theapparatus 21 in a substantially vertical orientation to connect to theforce measurement device 45, as shown in FIG. 3.

Referring again to FIG. 1, the tensile testing machine 25 includes alinear drive mechanism, generally indicated 87, adapted for moving theforce measurement device 45 along a linear path and actuating thelinear-to-rotary drive mechanism 71. In the preferred embodiment, thelinear drive mechanism 87 includes the movable crossbar 33 of thetensile testing machine 25, upon which the force measurement device 45mounts by conventional mechanical fasteners (not shown). Other suitablelinear drive mechanisms 87 are also contemplated as within the scope ofthe present invention. Upon linear movement of the linear drivemechanism 87, the linear-to-rotary drive mechanism 71 rotates the cam61. More specifically, movement of the crossbar 33 in an upwarddirection pulls the force measurement device 45 and the driving cord 75upward, causing the drive pulley 79, shaft 57 and cam 61 to rotatetogether in the direction of arrow R (FIGS. 3-5). The rate or speed ofrotation is predetermined in accordance with a rotational speed profile.In the preferred embodiment, the rotational speed profile provides asubstantially constant speed. Such a profile is readily achieved bymoving the crossbar 33 of the tensile testing machine 25 upward at aconstant speed, such that the driving cord 75 rotates the pulley 79 at aconstant angular speed. It is also contemplated within the scope of theinvention that a particular test procedure may require that therotational speed profile vary over time. Such a rotational speed profilemay be readily achieved by programming the tensile testing machine 25 tomove the crossbar 33 at a speed varying with time or by shaping thepulley 79 to have a shape other than circular, for example.

The apparatus 21 may further comprise one or more braking devices,generally indicated 95, operatively connected to the cam 61 forgenerating a braking force, or retrograde force, resisting rotation ofthe cam (FIGS. 3-5). In one instance, the braking device 95′ comprises afollower 99 biased into contact with a surface 103 of the cam 61 andimparting a force against the surface. Friction between the follower 99and the moving cam 61 creates the braking force resisting cam rotation.The braking force opposing rotation of the cam 61 varies according tothe friction between the cam surface 103 and the follower 99 and thenormal force component of the follower on the cam. As will be describedin greater detail below, the normal force component varies according tothe shape of the cam 61. Other braking devices 95′ are also contemplatedas within the scope of the present invention, such as magnetic,electromagnetic or electrostatic brakes.

The cam surface 103 of the braking device 95′ is shaped to vary thebraking force exerted by the follower 99 over time to simulate thedesired dynamic force event. In one embodiment, the cam surface 103includes a segment having teeth 107 and the follower 99 comprises a pawl(also designated 99 for convenience) adapted to contact the teeth.Preferably, the teeth 107 are of uniform size and are evenly spacedalong the cam surface 103. Such a cam surface 103 additionally includesa toothless segment 111 sized and shaped to remain free of contact withthe pawl 99. The cam surface 103 may be shaped to have otherconfigurations that simulate different dynamic force events, such as theserrated, simulated dynamic force response generally indicated 115 inFIG. 7. A serrated cam surface as shown in the Figures is not alwaysdesirable. Depending upon the dynamic force event sought to besimulated, the shape of the cam 61 may be adjusted accordingly. Forexample, the teeth may be sized and spaced non-uniformly, such that thesimulated dynamic force response 115 created by the pawl 99 and the camsurface 103 more closely resembles the locally erratic behaviorexhibited in a conventional peel test. The cam surface 103 may also beoriented other than parallel to the rotational axis of the cam 61 (e.g.,perpendicular to the rotational axis of the cam).

In the embodiment shown in FIGS. 3-5, the pawl 99 is mounted on afollower support 151 pinned at 155 to the support 53 b, the followersupport 151 being adapted to pivot between a “follower ready” positionshown in FIGS. 3 and 4 in which the pawl is positioned for engagementwith the cam 61 and a retracted position shown in FIG. 5 in which thepawl is remote from the cam. The follower support 151 is held in its“follower ready” position by a suitable device, such as a detent 159,mounted on the support 53 b and having an end receivable in a notch 165in the follower support. Preferably, the detent 159 is pinned at 161 tothe support 53 b so that it can be manually or automatically pivoted outof the notch 165 to allow the follower support 151 to move down to itsretracted position, preferably under the force of gravity. The pawl 99is mounted on a pivot 167 on the follower support 151 for pivotalmovement of the follower pawl relative to the support and to the cam 61.

A spring 177 biases the follower 99 (e.g., pawl) into contact with thecam surface 103. In the embodiment shown in FIGS. 3-5, the spring 177comprises a torsion spring, although other springs, such as compression,extension or leaf springs, for example, are also contemplated as withinthe scope of the invention. The torsion spring 177 includes a first end177 a engaging a first pin 179 and a second end 177 b engaging thefollower 99 itself. The torsion spring 177 biases the follower 99clockwise about pivot 167 to engage the cam 61. A stop 173 limitsmovement of the follower 99, such that when the toothless segment 111 ofthe cam 61 is adjacent the follower, the follower is spaced away fromthe cam surface 103 and cannot engage the cam (FIG. 3). This limit onthe movement of the follower 99 allows the force measurement device 45to measure the braking force on the cam 61 without the follower engaged,the importance of which will be discussed below. The normal forcecomponent imparted by the follower 99 on the cam surface 103 isdependent upon a distance D between axis C of the cam and a contactpoint P of the follower against the cam 61 (FIG. 4). Similarly, thebraking force exerted by the follower 99 on the cam 61 as measured bythe force measurement device 45 changes over time as the distance Dchanges. It should be noted that the force measurement device 45 may beplaced at various locations for operable connection with the apparatus21 without departing from the scope of the invention. For instance, atorque gage mounted on the shaft 57 of the apparatus 21 measuring ashaft torsional moment would provide an accurate dynamic force response.

Another braking device 95″ of the present invention preferably furthercomprises a tensioning device, generally indicated 187, mounted on theprimary support 53 b and operatively connected to the cam 61 (FIGS. 2and 6). In one embodiment, a braking cord 178 (wire, cable or otherflexible line) operatively connects the cam 61 and the tensioning device187. More specifically, a first end 179 of the cord 178 connects to abraking pulley 181 rotationally coupled to the cam 61 via the shaft 57.A second end 185 of the cord connects to the tensioning device 187. Inthe preferred embodiment, the braking force created by such a tensioningdevice 187 is substantially constant. Such tensioning devices are wellknown in the art. For example, the tensioning device 187 is preferably aConstant-Force Spring-Powered Return Reel, rated at 0.37 pounds, ModelNo. 61115A1, available from McMaster-Carr Supply of Chicago, Ill. Othertensioning devices, having different force ratings for instance, mayalso be used without departing from the scope of the present invention.In another configuration, the tensioning device comprises a mass (notshown) freely suspended by the braking cord 178 for creating agravity-induced tension in the braking cord. The tensioning device 187described above and creating a substantially constant braking force isdesigned to simulate a mass suspended from a cord.

In use, the apparatus 21 of the present invention operates to simulatedynamic force events. To perform such a simulation, the apparatus 21 ispreferably manually set to the configuration shown in FIGS. 2 and 3,i.e., to a configuration where the follower support 151 is held in its“follower ready” position to position the pawl 99 for engagement withthe cam 61, and the tensioning device 187 is properly connected to thepulley 181. With the follower pawl 99 in close proximity to thetoothless segment 111 (FIG. 3), the tensile testing machine 25 isoperated to move the crossbar 33 upward, causing the cam 61 and shaft 57to rotate. The rotational speed profile of the cam 61 is determined bythe speed of the crossbar 33, which can be controlled to provide thedesired cam rotational speed profile. Once the crossbar 33 is set inmotion, the remainder of the test process may be fully automated.

As the cam 61 rotates through its initial arc, the force required torotate the cam is measured by the force measurement device 45 to providea baseline force 197 (FIG. 7) based solely upon the braking forceapplied by the tensioning device 187, without braking forcecontributions from the follower 99 and cam, which are not yet engaged.After the toothless segment 111 rotates past the follower 99, thefollower pawl contacts successive teeth 107 on the cam surface 103 (FIG.4). As described above, the follower 99 is urged by spring 177 againstthe cam surface 103 and imparts a force against the surface.(Alternatively, the pawl 99 could be urged against the cam 61 bygravity.) As the cam 61 rotates, the normal component of this forceincreases as distance D increases and the follower 99 moves further fromaxis C along each tooth 107. As the contact point P of the pawl passesover the peak of each tooth 107, the normal force component on the cam61 quickly drops as the pawl moves toward axis C and distance Ddecreases. As the pawl 99 passes over successive teeth 107 of the cam61, the pattern of increasing and decreasing forces as measured by theforce measurement device 45 creates a sawtooth pattern 199 in themeasured braking force acting upon the cam 61 (e.g., FIG. 7). Followinga complete revolution of the cam 61, a protuberance 201 on the camengages the detent 159 and rotates it counter-clockwise until the detentexits the notch 165, allowing the follower support 151 to pivot downunder the force of gravity about pin 155 to disengage the follower 99from the cam 61. Without the force of the follower 99 on the cam, themeasured braking force returns to the baseline force 197 and datacollection ends.

The present invention further comprises a method of dynamicallycalibrating multiple measured characteristics (e.g., force,displacement, speed control) of a testing machine simultaneously, suchas a tensile testing machine 25, by collecting braking forcemeasurements generally as described above. Before a testing machine canbe properly calibrated according to the present invention, however, twoother steps must occur. The steps include establishing a testing machineknown to operate properly and creating a standard test based upon thattesting machine. By combining these two steps with the method ofcalibrating, a calibration process is defined. The steps will now bedescribed in detail with reference to a tensile testing machine 25,although they are generally useful when creating a calibration protocolfor any testing machine.

First, one skilled in the art establishes that a particular tensiletesting machine 25 is operating properly. For example, a tensile testingmachine 25 recently maintained, statically calibrated and accuratelyperforming peel tests with test samples may be classified as operatingproperly. Tensile testing machines 25 need not necessarily exhibit allthree of these characteristics to operate properly, however; one skilledin the art may consider different or additional criteria, depending uponthe specific tensile testing machine or test to be performed.

Second, apparatus 21 of the present invention is mounted (installed) onthe tensile testing machine 25 which has been established as operatingproperly. The tensile testing machine 25 then cycles through an entiretest with the apparatus 21 as described above. The braking forces on thecam 61 are measured and collected over time to establish a standarddynamic force response 205 (FIG. 8). The response depicted in FIG. 8 isillustrative only, and is submitted as an example of such a standard. Itshould be readily apparent to one skilled in the art that altering theapparatus 21 or tensile testing machine 25 could alter the details ofsuch a response, without departing from the scope of the invention. Onceestablished, the standard dynamic force response 205 may then beutilized to calibrate other tensile testing machines 25, where theoperational characteristics of the machines are unknown.

Third, now that the standard dynamic force response 205 is established,tensile testing machines 25 of unknown operational quality may becalibrated according to the standard. In one scenario, the tensiletesting machine 25 of unknown operational quality may be the samemachine used to develop the standard, but at a time remote from thestandard establishing test, when the machine may or may not be operatingproperly (e.g., after sufficient time has passed or a large number oftests have occurred). In another scenario, the standard may be developedon a first tensile testing machine 25 and applied to a second tensiletesting machine. In any event, the apparatus 21 is installed on thetensile testing machine 25 of unknown operational quality and performs atest identical to the one used to establish the standard dynamic forceresponse 205. The braking force on the cam 61 is measured and collectedover time to produce the simulated dynamic force response 115, ratherthan the standard dynamic force response 205. The simulated dynamicforce response 115 is then compared to the standard dynamic forceresponse 205 to calibrate the tensile testing machine 25.

Where a comparison of the simulated dynamic force response 115 and thestandard dynamic force response 205 reveals that the two responses sharesubstantially identical shapes and characteristics, the accuracy of thetensile testing machine 25 and the force measuring device 45 isconfirmed. However, if the comparison reveals substantial differencesbetween the responses 115,205, then the tensile testing machine 25should be calibrated. In addition, the tensile testing machine 25 itselfshould undergo routine maintenance before retesting. In other words, thecalibration method determines whether the tensile testing machine 25 ofunknown operational quality is performing accurately by comparing thesimulated dynamic force response of such machine with a standard dynamicforce response produced by a tensile testing machine functioningproperly. If the tensile testing machine 25 cannot emulate the standardresponse, then the machine is not operating properly.

When comparing the simulated dynamic force response 115 and the standarddynamic force response 205, any number of comparisons may be made todetermine the performance of the tensile testing machine 25 of unknownoperational quality. For instance, particular characteristics of theresponses may be compared to calibrate the tensile testing machine 25.More specifically, the mean forces of the dynamic force responses115,205 may be compared to determine if the measured forces provided bythe tensile testing machine 25 of unknown operational quality aresimilar in magnitude to those of the standard dynamic force response.Alternatively, the standard deviation of the force readings of thesimulated dynamic force response 115 and the standard dynamic forceresponse 205 may be compared to determine if the measured forcesprovided by the tensile testing machine 25 of unknown operationalquality are grouped as tightly together as those of the standard dynamicforce response. Finally, scatter plots of the force measurements (e.g.,FIGS. 7 and 8) of the simulated dynamic force response 115 and thestandard dynamic force response 205 may be compared to determine if themeasured forces of the tensile testing machine 25 of unknown operationalquality vary over time in a manner substantially similar to those of thestandard dynamic force response. Other statistical comparisons usefulwhen comparing two sets of data may also be used to compare thesimulated dynamic force response 115 and the standard dynamic forceresponse 205 without departing from the scope of the present invention.

While the apparatus described above may include a cam and a follower, itwill be understood that the present invention is not limited to thistype of mechanism, and that other mechanisms may be used to simulate adynamic force response. In general, apparatus 21 of this inventioncomprises a first body movable according to a specified speed profileand a second body biased into contact with a surface on the first bodyto impart a force against the surface. More generally, the first andsecond bodies need only move relative to one another, such that thesecond body may also be the moving body. A measurement deviceoperatively connects to the first body (or moving body) to measure thedynamic force response on the first body as the first body movesaccording to the speed profile. The force varies according to the forceof the second body on the first body and the friction between thesurface and the second body as they move against one another. Theapparatus preferably further comprises an additional braking deviceoperatively connected to the first body to apply a braking forceresisting movement of the first body. Such a braking force may besubstantially constant, as produced by a constant tensioning device asset forth above.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a,” “an,” “the,” and “said” areintended to mean that there are one or more of the elements. The terms“comprising,” “including,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

1. A method of simulating a dynamic force event comprising: rotating acam about an axis according to a rotational speed profile; biasing afollower against a surface of the cam as the cam rotates so that thefollower moves on the surface and imparts a braking force against saidsurface; and measuring the braking force on said rotating cam with ameasurement device operatively connected to said cam, said braking forcevarying according to the friction between said cam surface and followermoving against one another.
 2. A method as set forth in claim 1 furthercomprising applying a constant component of braking force resistingrotation of said cam.
 3. A method as set forth in claim 1 furthercomprising recording the braking force on said cam as measured by themeasurement device, over time.
 4. A method of calibrating a testingmachine adapted for measuring a force applied to a test specimen, saidforce varying over time to provide a simulated dynamic force, saidmethod comprising: moving first and second bodies relative to oneanother; biasing the second body against a contoured surface of thefirst body during said movement to impart a force against the firstbody, said contoured surface being shaped to vary the force appliedagainst the first body to provide the simulated dynamic force; measuringa braking force resisting movement of the first body over time toproduce a simulated dynamic force response; and comparing the simulateddynamic force response to a standard dynamic force response to calibratethe testing machine.